The present invention relates to a color printer which forms registered plural image exposures on a photoreceptor belt by a plurality of imagers and, more particularly, to improvements in printer efficiency and belt life by use of control means associated with detection of holes formed in the belt for registration and seam sensing purposes.
In single pass electrophotographic color printers having more than one process station which provide sequential images to form a composite image, critical control of the registration of each of the sequenced images is required. Failure to achieve registration of the images yields printed copies in which the images are misaligned and unsuitable for intended uses. In a single pass, color xerographic system, in which a plurality of imaging stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth imaging station is added if black images are to be created as well. Thus, each image area on the photoreceptor surface must make at least three passes relative to the transverse scan lines formed by a modulated laser beam generated by the imaging system. Each image is formed by a series of horizontal scan lines with each line having a certain number of pixels per inch. Each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (process registration) and in the direction parallel to the process direction (referred to as the fast scan or transverse registration).
Prior art color printers have utilized, as the imaging stations, raster output scanners (ROSs) or linear image print bars, typically LED print bars. A preferred method of obtaining the requisite color image registration is to form holes in the belt in non-image areas, the hole spaced at some known distance preceding one of the pitches (image exposure frames) of the belt. Each belt pitch has an associated belt hole with some systems providing pairs of belt holes formed at opposite sides of the belt for lateral process and skew registration purposes. As the photoreceptor belt moves through the various xerographic stations, the belt holes are detected by sensors output signals are used to provide the page sync triggering signals which expose the first and successive lines formed on the associated pitch. Other signals control the lateral registration (first information pixel of each scan line). The sensed hole pairs produce signals which are used to control for skew of the belt and/or the imager.
Prior art techniques for color registration utilizing belt hole detection in an LED printer system are disclosed in U.S. Pat. No. 5,040,003 and in co-pending application U.S. Ser. No. 07/862,150 filed Apr. 2, 1992 and assigned to the same assignee as the present invention.
Techniques for color registration utilizing belt hole detection in ROS-type systems are disclosed in U.S. Pat. No. 5,175,570 and in co-pending application U.S. Ser. No. 07/807,927 filed on Apr. 2, 1992 and 07/970,889 filed on Nov. 3, 1992, both applications assigned to the same assignee as the present invention. The above identified patents and applications are hereby incorporated by reference.
A problem in the prior art color registration systems described above is that the first image can only be formed when a belt seam is first detected. As is known in the art, belts are formed by a process which leaves a seam extending across the belt width. The seam presents a discontinuity in the photoreceptor surface. In operation the photoreceptor belt is moved at a predefined velocity, and the rate of travel of the advancing copy sheet is controlled so as to regulate the exposure and transfer operations in accordance with the position of the advancing sheet. Minor variations in the speed of the belt drive motor, due, for example, to variations in the power line voltage, result in a variation of the position of latent images on the photoreceptor. These variations are cumulative in nature and must be corrected to assure that the latent images are exposed at generally the same positions on the photoreceptor each time. If not corrected, the cumulative variation would eventually cause one or more of the exposed latent image areas to encroach on the photoreceptor seam, subsequently resulting in an unacceptable copy. A number of techniques can be used to provide seam sensing. For example, notches can be formed in the belt edge at known distances from the belt seam and detected by sensors which generate outputs used for timing and control purposes. Toner registration marks can also be formed along the edge of the belt and/or interframe area and detected by light passing through the belt and falling on dedicated sensor arrays. The detected marks result in sensor signal outputs which are used to identify the location of the seam.
With any of the seam sensing techniques, the first copy out time is a function of the seam location at the start of the print operation. It could be slightly less than a full belt revolution from the seam sensor, thus maximizing the first copy out time. A second disadvantage with the prior art seam sensing is that the first pitch or two downstream from the seam detection have much greater usage than pitches further downstream, e.g. pitches 3 and 4 in a 4 pitch, A3 copy system. Thus, the photoreceptor belt life is determined mainly by the wear and stress placed on the same two pitches of the belt. It would therefore be desirable to reduce the first copy out time interval and also to provide a random pitch imaging to prolong belt life where the first pitch to be imaged is not necessarily the pitch following the seam detection.
The present invention is directed towards an improved pitch registration and seam detection circuitry in a color printer which utilizes in one embodiment a plurality of holes associated with a plurality of image exposure frames or pitches on the circumference of a photoreceptor belt. In this embodiment, one of the holes is used for both identification of the seam and as a timing hole for sensing and registering an associated downstream pitch. The hole is formed slightly different than the other pitches; in the embodiment shown, by making it 50% longer than the other pitch holes in the process or belt travel direction. The printing operation can then begin immediately with the detection of a first pitch hole which passes by the sensor associated with the first imager (not necessarily the belt hole associated with the seam). Thus, the first pitch to be imaged will also be random. This embodiment will be able to accommodate the printing of A4 (81/2.times.11) and A3 (11.times.17) images as will be seen. In a second embodiment which can be used in machines dedicated to printing of A4 documents, control electronics are provided which treat each detected hole as if it were associated with a seam and provide delay such that each pitch associated with each detected hole receives a page sync signal following the same time delay. As will be explained, marks can be formed on a photoreceptor surface and used in the same way as the holes. Holes or marks are referred to generically as "targets". More particularly, the present invention relates to an electrophotographic printing machine which incorporates a photoreceptor belt which moves along a preselected path in a process direction, said belt having a seam extending across the width thereof, said color images formed in overlying registration to form a composite color image,
means for forming a plurality of targets on the surface of said belt, each of said targets associated with one of said belt pitches, one of said targets having a size different from the other marks, said one of said targets further associated with identification of said seam,
means for sensing said targets and for generating output signals representing detection of said targets, and
control means response to said output signals for maintaining the overlying registration of said color images and for preventing said images from overlying said seam.